GALEX far-ultraviolet color selection of UV-bright high-redshift quasars.
WORSECK G. and PROCHASKA J.X.
Abstract (from CDS):
We study the small population of high-redshift (zem>2.7) quasars detected by the Galaxy Evolution Explorer(GALEX), whose far-UV emission is not extinguished by intervening H I Lyman limit systems. These quasars are of particular importance to detect intergalactic He II absorption along their sight lines. We correlate almost all verified zem>2.7 quasars to the GALEX GR4 source catalog covering ∼ 25,000 deg2, yielding 304 sources detected at signal-to-noise ratio (S/N) >3. However, ∼50% of these are only detected in the GALEX NUV band, signaling the truncation of the FUV flux by low-redshift optically thick Lyman limit systems. We exploit the GALEX UV color mFUV - mNUV to cull the most promising targets for follow-up studies, with blue (red) GALEX colors indicating transparent (opaque) sight lines. Extensive Monte Carlo simulations indicate an He II detection rate of ∼60% for quasars with mFUV- mNUV ≲ 1 at zem ≲ 3.5, a ∼50% increase over GALEX searches that do not include color information. We regard 52 quasars detected at S/N >3 to be most promising for Hubble Space Telescope follow-up, with an additional 114 quasars if we consider S/N >2 detections in the FUV. Combining the statistical properties of H I absorbers with the Sloan Digital Sky Survey (SDSS) quasar luminosity function, we predict a large all-sky population of ∼200 quasars with zem>2.7 and i ≲ 19 that should be detectable at the He II edge at m304< 21. However, SDSS provides just half of the NUV-bright quasars that should have been detected by SDSS and GALEX. With mock quasar photometry we revise the SDSS quasar selection function, finding that SDSS systematically misses quasars with blue u - g ≲ 2 colors at 3 ≲ zem ≲ 3.5 due to overlap with the stellar locus in color space. Our color-dependent SDSS selection function naturally explains the inhomogeneous u - g color distribution of SDSS DR7 quasars as a function of redshift and the color difference between color-selected and radio-selected SDSS quasars. Moreover, it yields excellent agreement between the observed and the predicted number of GALEX UV-bright SDSS quasars. We confirm our previous claims that SDSS preferentially selects 3 ≲ zem ≲ 3.5 quasars with intervening H I Lyman limit systems. Our results imply that broadband optical color surveys for 3 ≲ zem ≲ 3.5 quasars have likely underestimated their space density by selecting intergalactic medium sight lines with an excess of strong H I absorbers.